Approximately 100 million pounds of energetic material waste, or 41 percent of the total life-cycle energetic waste production, is generated during munitions manufacture. An additional 449,000 tons of energetic waste is produced during demilitarization of these munitions. State-of-the-art propellant and explosive formulations are typically cast-cured composite materials mixed in batches ranging from a few gallons to hundreds of gallons. The ullage and overage material produced as a result of these processes forms a large proportion of the resultant waste.
Polyurethane linkages are widely used to polymerize the binders in both propellants and plastic bonded explosives (PBX) via the reaction of long-chain, high-molecular-weight, oligomeric polyalcohols with polyisocyanates. Isocyanates begin to react with any available hydroxyl groups in or on the binder oligomers upon mixing. These hydroxyl groups can include those available from trace amounts of moisture in the binder or the ingredients. The reaction rate is increased, preferentially with the organic hydroxy groups, by the addition of catalysts, promoters or heat, but it runs at a finite rate regardless of the concentrations of these additives or the ambient temperature. These mix parameters are adjusted and balanced against each other in the processing facility to optimize two essentially contradictory aspects of the mixture: a maximum pot life, in order to have plenty of time to mix and cast the formula into the item needed; and, a minimum cure temperature and time, which allows a rapid turnover of finished items with a minimum of thermal damage potential to what may be sensitive and/or reactive ingredients.
However, once the curative is mixed in, there is no practical way to stop the reaction. Consequently, if excess material has been made, and it can not be used that day and the mixture must be disposed of, as it will either be too viscous to remix with fresh formulation, or the cure reaction will have progressed or degraded by homopolymerization and reactions with moisture to the point where its use or addition to fresh formulation would only degrade the processability and ultimate properties of the freshly-made propellant or explosive.
As a consequence of this inexorable cure reaction, leftover material and material clinging to the sides of the mix pot or the casting hardware after casting must typically be cleaned up and disposed of. The cleanup process generally involves a laborious scraping down and wiping of the surfaces by hand, the use of solvents that may be flammable or toxic or both, and the transfer of the excess mixture, together with the wipes and contaminated solvent, to proper containers for disposal. The costs of the disposal of this hazardous energetic material are increasing steadily, not only in costs for permits and fees charged by authorized disposal sites, but also in the lost value of the ingredients and binders thus disposed of. The environmental burden of the disposal is also increased by the loss of the ingredients, since environmentally unfriendly processes must frequently be resorted to in order to replenish stocks of these energetic ingredients.
The reclaim and reuse of such waste or “scrap” energetic material is thus seen as a desirable goal. Some binders have been specially synthesized so that they can be hydrolyzed by dilute acids or bases and the solid ingredients recovered by recrystallization or filtration as illustrated in U.S. Pat. No. 4,293,352. The method that perhaps has achieved the greatest application in actual production is the use of thermoplastic elastomeric materials as binders. In principle, these formulations can be recovered by melting or dissolving the TPE binder away from the solid ingredients, reconstituting the binder with added TPE, and reformulating. However, this recovery process is only practical with a narrow range of binder materials, and the processing must be done with an extruder, which means that processing is very much less facile and adaptable than by using the cast-cure processes described earlier.
From the foregoing, it will be appreciated that there is a need in the art for a cast-cure process with a cure reaction, that, for all practical purposes, can be stopped and started at will. Such a process will allow leftover quantities of formulation mixtures to be stored and recycled into new mixtures of the same formulations. This recyclability will result in a great reduction of the amount of hazardous waste generated by a mixing facility, with a concomitant reduction in the expense, hazards, pollution, and environmental burdens associated with this waste.